Question

I am using curl library which returns data to me through a callback function with the prototype below

size_t write_data(void * data, size_t size, size_t nmemb, void * userpointer);

I noticed that if i declare a function fitting this prototype in my class

//file Dialog.h
class Dialog : public QDialog
{
    private:
    int new_data_callback(void * newdata, size_t size, size_t nmemb, QByteArray * buffer); 
}

If i try to use it in my Dialog.cpp

curl_easy_setopt(curl_handle,CURLOPT_WRITEFUNCTIION, new_data_callback);

I get an error

Invaid use of member (did you forget the '&'?)

If i add static to my function declaration, it compiles.

static int new_data_callback(void * newdata, size_t size, size_t nmemb, QByteArray * buffer); //ok

Question

Why is static needed in this case?

PS: the classes beginning with Q eg QDialog are part of QT and dont affect the question.

Answer 1

Because a non-static method cannot be called without an instance. Since new_data_callback is a callback, the only way to attach an instance to it is through a parameter. Making it static removes the instance restriction.